The present invention relates to control over an AC-DC converter system provided in a DC power transmission system or in a frequency converting system, in which two AC power systems are coupled by an AC-DC converter system comprising a pair of converters and an intermediate DC line connecting the converters.
With conventional control systems for an AC-DC converter system the voltage of a healthy or sound AC power system is subject to variation when a fault occurs in another AC power system. The voltage variation is not too large when the healthy power system is strong, i.e., its short-circuit capacity is large relative to the capacity of the converter. But the voltage variation is substantial when the sound power system is weak. The voltage variation may become so large as to cause a breakdown of the system.